The proposed studies attack unresolved questions about the fundamental organization of the cochlear spiral ganglion and its input to the brainstem. Using the cat as the animal model, certain inadequately substantiated concepts of the neuronal wiring of the mature cochlea will be re-examined. A combination of auditory evoked response, physiological microelectrode recording and modern tracer techniques along with extensive light and electron microscopic analyses will be employed. Special attention will be given to the morphology, distribution and connections of the small unmyelinated "Type II" ganglion cells contrasted with the more common "Type I" cells. The nature of neurons which resist retrograde degeneration following transection of the cochlear nerve or long term total or partial destruction of the organ of Corti by neomycin sulfate will be re-evaluated. The central distribution of afferents from the two hair cell populations via their respective parent ganglion cells will be defined using modern neuroanatomic tracer methods. Further development and documentation of a reproducible animal model which has large sectors of the organ of corti with inner outer hair cells selectively preserved following ototoxic drug treatment will be provided.